Should Stakeholders/Product Owners come to sprint retrospectives or sprint reviews?

Firstly, the Sprint Review and sprint retrospective are not one in the same. They have different roles and different goals.

Who Attends The Sprint Review?

The sprint review should be attended by anyone involved in the processes in the last two weeks of product development (which includes stakeholders). Here, the group has a look at the project and talks about what they need to change about the project regarding the work done in the last iteration. In essence, it looks at what was produced, not the methods behind how it was made.

Although the Product Owner  can represent stakeholders in the sprint review, the stakeholders should be present themselves, to see with their own eyes what was worked on. However, it must be reinforced to them what their role is - an observer. It is the Product Owner (and Scrum Master’s) responsibility to make sure they know how to act.

Who Attends The Sprint Retrospective?

The sprint retrospective on the other hand is predominantly for the team to have a look back over their last sprint at how it was done, paying less attention to output. This means that only people involved in the building part of the sprint will find it useful, and any extra attendees might actually make it difficult for the team to share openly and truthfully. The Product Owner can be included if it’s their preference but any other outside personnel isn’t recommended.

However, perhaps running a retrospective for management over a  longer time period could be effective, for example every quarter. This allows management to stay aligned with the team's productivity and their contribution to the ‘bigger picture’ of the company/project.